The incidence of obesity and metabolic syndrome X are increasing towards epidemic proportions in the US and developed world. Breastfeeding provides modest protection towards development of childhood and adult obesity presumably through a perinatal metabolic programming mechanism. Exclusive breastfeeding is recommended for the first 6 months of life, but is not currently achieved because of a number of factors including mother<s perception of her infant<s satiety and her ability to meet milk demand. Milk fat is the major energy component of breast milk and is important to infant satiety and some of the documented healthful properties of breast milk. However, milk fat concentration and fatty acid (FA) profile are the most variable components of breast milk. Breast milk fat concentration is different between women, the proportion of de novo synthesized FA is decreased by substitution of dietary fat for carbohydrate, and milk fat concentration and profile follows a circadian rhythm on some diets. While investigating the functional role of Thyroid Hormone Responsive Spot 14 (S14) in trans-10, cis-12 conjugated linoleic acid (CLA) inhibition of milk fat synthesis we observed that the S14 null mouse was hyper-responsive to this bioactive FA. Thus, we proposed that S14 expression is important to regulation of lipogenesis by modifying the responsiveness or effectiveness of a second lipogenic regulatory pathway. Based on this observation and differences in the phenotype of the S14 null mouse on different chow diets we propose the hypothesis that milk fat synthesis in the S14 null mouse is hyper-responsive to dietary factors. We believe this represents a novel nutrient-gene interaction and a new paradigm of the functional role of S14 in regulation of lipid synthesis by dietary factors. The proposed work will test the effect of dietary factors on mammary lipid metabolism during established lactation. Specifically, we will investigate the role of S14 in milk fat response to dietary fat concentration and FA profile, time of day, and short-term feed restriction. Main response variables include pup growth, dam feed intake, and pup milk fat profile. Lastly, we will characterize the mechanism of the most responsive dietary treatments by determining mRNA and protein expression of key lipogenic enzymes and transcription factors. The proposed work will determine the functional role of S14 in milk fat response to dietary factors and has significant implications in development of recommendations on dietary fat content and meal timing in breast feeding women. The project is also expected to yield new insight into the role of S14 in metabolic regulation. )